Revamping Shaft Design for Combating Fatigue Failure

This paper gives insights on the failure analysis of the non-drive shaft of a conveyor in a coke plant of an integrated steel plant. The failure of the shaft was investigated in order to determine the root cause and contributing factors. Investigation methods included visual examination, optical and scanning electron microscope analyses, chemical analysis of the material, mechanical tests and life calculations based on design. The overall appearance of the fracture surface of the 45C8 grade shaft indicated rotating bending fatigue failure from a step location. Microstructural analysis revealed a crack from the sharp step and no metallurgical abnormalities. Apart from the metallurgical analysis, the design of shaft was also verified by calculating step profile, fillet radius and calculated stress in actual condition. It revealed that a shaft with a minimum diameter of 70 mm, made of 45C8 steel, can withstand the actual stress acting on it. In this design, the shaft diameter was found to be 65 mm which is insufficient to bear the actual stress. Shaft failures can be reduced by preventive mechanical maintenance and using safe design. To enhance the shaft life without an increase in shaft diameter, we recommend that the material be changed to 42CrMo4 grade steel, which, when appropriately heat treated, has higher stress limit, along with a modification in the step profile with a proper fillet radius.